Waste liquids in the environment may result from several sources, e.g. uncontrolled dumping of pure solvents, spills or infiltration of water through solid waste in landfill disposals resulting in contaminated leachate. Contaminants in this leachate can lead to significant damage to the environment and to the human health due to their mobility and solubility.
Clayey barriers are used to isolate pollutants from the environment. Bentonite clay is widely used in barriers for landfill systems because of its elevated sealing capacity in presence of water. However, exposure to leachate can cause loss of efficiency of the clay as hydraulic barrier with consequent harm to the environment. Clays have therefore been engineered in order to improve their chemical and physical properties.
Extensive research has been conducted to characterize the sorption of organic compounds onto clay surfaces (Lo et al. 1997; Lorenzetti et al. 2005; Bartelt-Hunt et al. 2005). Organobentonites are clays, typically amended by exchanging quaternary ammonium cations for the naturally occurring sodium. This process renders the modified clay hydrophobic and highly organophilic. Organically modified clays have been found to be a promising to resist pollutant transport (Lo et al. 1997). Bentonites modified with ammonium compounds (organoclays) have sorption capacities for organic compounds 4-5 times higher than untreated clays (Lorenzetti et al., 2005). However, the hydraulic conductivity of organoclays—an essential characteristic for being a performing clayey barrier—increases significantly upon modification with organics.
Multi-Swellable Bentonite (MSB), developed by Kondo (1996), is a bentonite which has been mixed with Propylene Carbonate (PC) to activate the osmotic swelling capacity. Propylene Carbonate is placed in the interlayer of the smectite and attracts numerous water molecules. This results in a strong swelling ability even if the permeant contains polyvalent cations or a high concentration of monovalent cations (Onikata et al., 1999). The hydraulic conductivity of MSB is one to two orders of magnitude lower than that of the untreated clay (Katsumi et al., 2008). On the other hand, after prolonged prehydration with water, the hydraulic conductivity of the MSB increases one order of magnitude due to the release of the PC during prehydration due to the weak bound of the PC to the clay (Mazzieri and Pasqualini, 2006; Mazzieri et al., 2010). Moreover, swell index tests were performed on the MSB specimen at the end of a chemico-osmotic/diffusion test (Mazzieri et al. 2010). The latter tests demonstrated that the MSB converted into a Ca-exchanged bentonite which is partially or totally deprived of PC and of the swelling properties initially conferred by PC.
U.S. Pat.No. 6,537,676 discloses a dense prehydrated geosynthetic clay liner (DPH GCL) which is densified by calendering after prehydration with a polymeric solution containing sodium carboxymethyl cellulose (Na-CMC), sodium polyacrylate, methanol and other ingredients. This DPH GCL performs well in various aggressive solutions (Schroeder et al. 2001, Kolstad et al. 2004, Di Emidio et al., 2008, Katsumi, 2008). Polyacrylate compounds in this DPH GCL can replace the sodium cations of the clay to avoid ion exchange, enhancing the impermeable behavior under aggressive conditions (Flynn and Carter, 1998). However, DPH GCL contains numerous components which complicates its production. Moreover, the polymer adsorption on to the DPH GCL occurs without dehydration at high temperatures. This method of preparation may release the polymers in the long term. Mazzieri and Pasqualini (2008) studied the permeability of the DPH GCL subjected to dry/wet cycles and using a 12.5 mM CaCl2 solution as hydrating liquid. They observed that the polymer was probably removed during the test.
Cationic polymers dissolved in a solution may adsorb easily to both sand and clay surfaces (Stumm, 1992). Such adsorption in clays can be irreversible and entropy-driven because a cationic polymer chain displaces many water molecules and contains thousands of cations which would need to be displaced simultaneously (Theng, 1982; Ashmawy et al., 2002). For this reason cationic polymers can protect the clay from cation exchange, that is the main reason for the increase of permeability. However, cationic polymer amendments provide no improvement to the hydraulic conductivity of bentonites (McRory and Ashmawy, 2005). The reason of this behavior is probably due to the compression of the double layer thickness caused by the presence of the cationic polymers, and to the tendency of flocculation instead of dispersion.
U.S. Pat. Nos. 6,340,385 and 7,026,385 disclose a well-defined mixture of sand (lower than 89.1% by dry weight), bentonite (higher than 10.7% by dry weight) and a special polymer (higher than dry 0.2% by weight). However, these mixtures are not dehydrated before usage as a clay liner. On the other hand, a sand-bentonite-mixture used as compacted clay liner has already a good sealing property, which, together with other geotechnical properties, is considerably amended by the polymer additive (Simon and Müller, 2005). The synthetic additive is a high molecular weight, hydrophilic and gel-forming polymer. Therefore, all water transport processes in the mixture are strongly retarded by the polymer. In addition it gives rise to some internal cohesion in the sand-bentonite mixture. This clay liner has a hydraulic conductivity one order of magnitude lower than conventional compacted clay liners (Simon and Müller, 2005). However, its long-term performance is questionable as its method of preparation could lead to weak bonds. Indeed, the polymer (acrylamide) and the clay is mixed in dry conditions and this type of dry mixing lead to weaker bonds compared to drying the slurry of clay and polymers after wet-mixing.
WO90/13598 and U.S. Pat. No. 6,806,298 disclose a composition which can be used as a cover layer and comprises a polymer, clay and water. However, these composition are not dehydrated before they are used as a cover layer.
Di Emidio et al. 2008 (EuroGeo4, paper 320) describe the evaluation of a dense prehydrated geosynthetic clay liner comprising bentonite and the addition of polymers. However, also this clay liner is not dehydrated before it is used as a clay liner.
Stutzmann and Siffert (1977) compared the quality of the adsorption of anionic polymers on to montmorillonite surface for two scenarios: (a) drying the treated clay either at 60° C. and (b) drying under vacuum at 20° C. They found that the adsorption of the polymer on the montmorillonite after drying at 60° C. can be considered as intense, irreversible fixation, corresponding to chemisorption. On the other hand, the adsorption observed with vacuum drying corresponds rather to an unstable adsorption equilibrium, such as a reversible physisorptive adsorption. This study was undertaken to understand the retention of additives used in tertiary recovery of petroleum. However, these authors did clearly not demonstrate nor suggest that adsorption of the polymers influences the hydraulic performance of the clay.
Hence, it is clear that there is still a need for an industrially useful and easy method to engineer a clay which is well protected—and for a sufficiently long time period—from chemical attack by aggressive electrolyte solutions so that said it can be used as a clayey barrier having superior properties.